Relays used for switching suffer degradation caused by sparks created upon opening relays that are carrying current. The sparks cause erosion of the switch contacts, which damages the contacts over time. In addition, sparks may present other potentially dangerous conditions, especially if used in environments where combustible or explosive gases are present. One solution to the problem of spark generation is to open relay contacts at the time when the voltage applied to the contacts is zero volts, or so-called switching at zero crossing using zero-crossing detection circuits as described in U.S. Pat. No. 4,360,847, entitled “Diode Assisted Relay Contactor,” issued on Nov. 23, 1982 to Bloomer et al.
U.S. Pat. No. 5,530,615, entitled “Method and apparatus for enhancing relay life,” issued on Jun. 25, 1996, to Miller et al., describes opening or closing the contacts of a relay used in a furnace control system or a gas valve solenoid at or shortly before a zero crossing (e.g., zero voltage across the contacts) by measuring a relay mechanical time constant parameter and using this parameter to set fixed time delays for relay activation.
U.S. Pat. No. 4,321,946, entitled “Armature position monitoring and control device,” issued on Mar. 30, 1982 to Paulos et al., describes apparatus and methods for detecting the movement of the armature of a solenoid by monitoring the backward electromotive force (back EMF) induced in the solenoid coil by the movement of the armature while exposed to the residual field magnetic field of the solenoid coil. A first derivative of a current passing through the solenoid is used to identify the time when the back EMF is generated.
U.S. Pat. No. 6,233,132, entitled “Zero cross relay actuation method and system implementing same,” issued on May 15, 2001 to Jenski (hereinafter “the Jenski patent”) describes apparatus and methods operating relay contacts under zero crossing conditions by detecting slope changes in coil voltage and current. Jenski's method requires that a resistor be placed in parallel with the relay coil. Then upon de-energizing the relay coil a “unique” voltage appears, as presented by Jenski at FIG. 2, that may be used to predict the instant of contact opening. The instant of contact opening is found by means of detecting a change in slope for the voltage waveform shown in FIG. 2. In practice, the contacts usually open sometime after the point in time indicated by Jenski, dependent upon relay design. Jenski's method requires a simple slope detector circuit for proper operation. The system records the history of a particular relay, including turn on and turn off times as functions of both positive-going and negative-going portions of sine wave excitations, and uses the historical data to calculate when to actuate the relay contacts.
There is a need for apparatus and methods that will provide accurate timing to control the actuation of relays contacts at a time selected to provide zero voltage switching (or alternatively, switching at a selected voltage) of the relay which is simple and inexpensive to implement and which improves the long term performance of the contacts relative to presently available technology.